Immunoassay is a field in which the sensitivity of the analysis method often is of decisive importance as the amount of analyte in different biological liquids usually is very low. As a result of this radioisotopes have been widely used as labels in immunoassays despite the disadvantages caused by their use. At the same time, however, a very intense research has been carried out with the aim of replacing the radioisotopes with labels giving at least the same or a higher sensitivity than the isotopes. Fluorescent molecules have in these connections been presented as one of the most potential alternatives to radioisotopes. Comprehensive surveys have recently been published, which give a good general view of fluoroimmunoanalytical determinations known at present (see Smith et al. (1981) Ann. Clin. Biochem. 18, 253-274, Ullman (1981) "Recent Advances in Fluoressence Immunoassay techniques").
The sensitivity of the fluorescent labels in immunoassay, in spite of the fact that it is theoretically very high, has been seriously limited by a high background fluorescence. Usually, it has been possible to reduce the background fluorescence, so that a desired sensitivity could be obtained. The above mentioned surveys also describe the limitations which have made the use of conventional fluorescent labels difficult in immunoassay of analytes which require a high sensitivity corresponding to that which can be obtained with radioisotopes.
The use of time-resolved fluorescence (see Soini et al (1979) Clin. Chem. 25, 353-361) makes it, however, possible to separate the specific fluorescence of the label from the disturbing, unspecific background fluorescence. The principle of the use of time-resolved fluorescence when following biospecific affinity reactions is described in the U.S. Pat. No. 4,374,120 and the European patent application No. 82850077.7. In time-resolved fluorescence the fluorescent label is excited by means of a light pulse of a short duration and the fluorescence is not detected until a certain period of time has elapsed from the excitation pulse. During the time which passes between excitation and detection, the fluorescence from any interfering sources will decay, so that only the signal from the label usable for time-resolved fluorescence is detected. Such a label should have as high fluorescence as possible, a relatively long emission wave-length, a large Stoke's shift and a chemical structure which makes it possible to couple the label covalently to antigens, haptens, antibodies, nucleic acids and polynucleotides. A fluorescence label, which fulfils the above mentioned requirements (U.S. Pat. No. 4,374,120) comprises a lanthanide chelate formed by a lanthanide and an aromatic .beta.-diketone, the lanthanide being bound to antigen, hapten, antibody, nucleic acid or polynucleotide via an EDTA-analogue so that a fluorescent lanthanide complex is formed. The fluorescence decay time of the label is long, 50-1000 .mu.sec, which makes it most suitable for the time-resolved detection principle. The fluorescence from the label can either be measured when the marker is bound to antigen, hapten, antibody, nucleic acid or polynucleotide, or the lanthanide can under suitably chosen circumstances be released from these by dissociating the bond between the lanthanide and the EDTA-analogue, the fluorescence being caused in solution in the presence of an aromtic .beta.-diketone, a synergistic compound and a detergent which together with the lanthanide form a micellar system having a fluorescence which is characteristic of the lanthanide (European patent application No. 82850077.7).
In the use of lanthanides as labels in biospecific affinity reactions two functions can in principle be distinguished. On the other hand, the lanthanide should form a fluorescent chelate and on the other hand it should be bound to a bio-organic molecule, which is an antigen, a hapten, an antibody, a nucleic acid or a polynucleotide, in order to be usable as a label in biospecific affinity reactions.
The prerequisites for the formation of a fluorescent lanthanide chelate are described in the European patent application No. 83850244.1. A specific controlled binding of a lanthanide to a bio-organic molecule has proved to be difficult even if a number of alternative solutions has been tested. In such a binding it is desirable that the lanthanide is bound to the bio-organic molecule with a very high affinity and that the binding is kinetically stable. The primary ligand which is covalently bound to the bio-organic molecule and which also chelates the lanthanide, can also absorb the excitation energy which is then transferred to the lanthanide according to the principles which are described in the European patent application No. 83850244.1, or alternatively the primary ligand only acts as an intermediary for the binding of the lanthanide to the bio-organic molecule. The EDTA analogue mentioned earlier (U.S. Pat. No. 4,374,120) follows the latter principle. Aminophenyl-EDTA-Eu can e.g. be diazotated and thereafter be coupled to tyrosine or histidine residues in a protein. The synthesized protein-EDTA-Eu complex gives, however, upon excitation a very low lanthanide fluorescence of a long decay time, since the primary ligand does not absorb and transfer the necessary excitation energy to the lanthanide. In spite of this the ligand functions excellently in bio-specific affinity reactions according to the principles which are described in the U.S. Pat. No. 4,374,120 and the European patent application No. 82850077.7.
Ethylenediamine tetraacetic acid (EDTA) is a well known and commonly used compound, which under the suitable conditions forms stable chelates with a large number of metal ions (see Ringbom (1964) Kompleksometrisk analys). The chelate forming characteristics of the molecule can be utilized to bind e.g. lanthanides to bio-organic molecules for use in bio-specific affinity reactions, if a covalent coupling of the ligand to the bio-organic molecule can be carried out. This has been done by e.g. synthesizing an EDTA dianhydride, which is coupled to a suitable bio-organic molecule, a diaminetriacetic acid derivative of the molecule is, then, obtained when the fourth carboxyl group is used for the conjugation (see Wieder et al, U.S. Pat. No. 4,353,751). Moreover, an aminophenyl-EDTA derivative can be synthesized which can be used to bind the EDTA structure to the bio-organic molecule (see Sundberg et al, U.S. Pat. No. 3,994,966).
There are, however, other compounds than EDTA which form stable chelates with metal ions (see Ringbom (1964) Kompleksometrisk analys). One of them, diethylenetriaminepentaacetic acid (DTPA) has e.g. been used to bind radioisotopes in connection with the examination of kidney functions (see Klopper et al. (1972) J. Nucl. Med. 13, 107-110). DTPA has also been coupled to protein by the use of a cyclic anhydride of the molecle, four carboxyl groups then remaining for chelating (see: Hnatowich et al. (1982) Int. J. Appl. Radiat. Insot. 33, 327-332).